The present invention relates to closed-loop ring resonators and methods for fabricating the same.
Evanescently coupled closed-loop ring resonators in general, and Racetrack Ring Resonators (R3s) or elongated loops in particular, offer several interesting features for applications such as Dense Wavelength Division Multiplexing (DWDM) and RF photonics. Lacking reflectors, fabrication of the same is generally free of lapping, cleaving, facet etching and coating, or of concerns arising from high mirror power density. Spectral characteristics of the same can be conventionally determined using well known photolithography techniques, and single wavelength operation can be achieved without gratings. When combined with other components small diameter low loss passive and active ring resonators, on the order of  less than 200 xcexcm, enable the fabrication of sophisticated photonic integrated circuits that take full advantage of two-dimensional (2-D) chip layouts. With the development of robust fabrication methods, such photonic circuits may include composite linking and switching systems, local oscillator distribution, and true-delay signal distribution at wavelengths of 1.3 and 1.55 xcexcm to name a few.
Previous efforts have been carried out to demonstrate R3 Lasers (R3Ls) in both AlGaAs and InGaAsP material systems and using different coupling schemes. Undesirably, conventional, shallow-etched Ridge-WaveGuide (RWG) circular structures are limited to large diameters, on the order of xe2x89xa7300 xcexcm, due to excessive bending loss. The low index difference between the guiding channel core and its surrounding of RWG structures causes a significant evanescent portion of a guided mode, which cannot propagate at group velocities exceeding
c/neff 
where c is the speed of light in vacuum and neff the effective refractive index of the mode to radiate. Even for large-diameter shallow-etched structures, significant bending loss is evidenced by large threshold currents ranging from 106 to 150 mA.
Coupling into deeply etched laser structures has been achieved by using either Y-junction or MultiMode Interference (MMI) couplers. In spite of the reduced bending loss, deeply etched R3Ls have also demonstrated large threshold current values, in excess of 170 mA. Vertical walls etched through an epitaxial waveguide have also been used in small diameter passive ring structures, on the order of xe2x89xa610 xcexcm, to allow negligible waveguide bending and scattering loss. However, strong confinement caused by deep etching undesirably requires sub-micrometer lateral separation between the ring and coupled waveguides to achieve adequate coupling and requires fabrication tolerances of xe2x88x9210/+20 nm. Furthermore, when used as a highly transmissive filter, coupling coefficients in and out of the ring resonator must be nearly identical, undesirably tightening fabrication constraints even further. Reliance on sub-micrometer features and tolerances entails significant impediments to achieving a robust manufacture-able technology that lends itself to large-scale integration and mass productive of ring resonator based photonic circuits.
It is an object of the present invention to overcome these prior art limitations and provide an improved closed-loop ring resonator and method for manufacturing the same.
A closed-loop ring resonator including a closed loop formed on a substrate and including at least one coupling region having a first effective depth and at least one other region having a second effective depth, wherein the first and second depths are different.